Now that the Supreme Court has upheld the health insurance mandate in the Affordable Care Act (ACA), two conservative scholars have come up with another legal argument for attacking health care reform. In a paper released Monday, Jonathan Adler and Michael Cannon argue that an IRS regulation implementing the ACA’s tax credits and cost-sharing subsidies is “illegal.”

The IRS rule provides credits and subsidies to those enrolled in new health insurance exchanges operated by the states or the federal government, but the scholars claim the ACA limits tax credits to those enrolled in state exchanges. Adler and Cannon argue that middle-class Americans enrolled in federal exchanges should not receive tax credits to help them afford health insurance.

If their argument was accepted by a court, governors would have the power to drastically undercut the ACA’s reforms. Some Republican governors have thus far refused to set up exchanges for their states. The federal government will step in to create exchanges in these states, but without subsidies and credits, the federal exchanges could be unworkable. The ACA’s preexisting condition rules and limits on setting premiums could lead to a rise in premiums, so the tax credits and subsidies are essential.